LWN: Comments on "A general caching filesystem"

A general caching filesystem

jcm — Mon, 25 Oct 2004 00:55:30 +0000

xoddam writes:

> This seems to me like a really complicated reimplementation of
> virtual memory.

No it's really not. By virtual memory your are referring to an aspect of VM implementations known as paging, and that in itself only really impacts upon so called ``anonymous memory''. There is a page cache for certain regular filesystems but it's not possible for all filesystems to exploit the page cache to full effect and in any case this patch adds the ability to use a local disk as an additional cache storage for even slower stuff like network mounted filesystems - so the page cache can always sit between this disk and user processes which use it.

Jon.

Improve "Laptop mode"

BrucePerens — Thu, 07 Oct 2004 18:57:31 +0000

I haven't looked at the CacheFS code yet, but this is what I would like to do with it, or something like it.

Put a cache filesystem on a FLASH disk plugged into my laptop. My laptop has a 512M MagicGate card, which looks like a USB disk. Use it to cache all recently read and written blocks from the hard disk, and allow the hard disk to remain spun down most of the time. Anytime the disk has to be spun up, flush any pending write blocks to it.

This would be an improvement over "laptop mode" in that it would not require system RAM and could thus be larger, and would not be as volatile as a RAM write cache.

Bruce

A general caching filesystem

xoddam — Tue, 14 Sep 2004 02:13:00 +0000

This seems to me like a really complicated reimplementation of virtual
memory.

All filesystems already use VM pages for caching, don't they?
I'd have thought that attaching backing store to those pages would have
been a much simpler task than writing a whole new cache interface.

But then I'm not really a filesystem hacker.

CacheFS & Security

AnswerGuy — Mon, 13 Sep 2004 18:49:07 +0000

The only difference between accessing a filesystem directly and through CacheFS should be that the CacheFS can store copies of the accessed data on a local block device. In other words that there's a (potentially persistent) footprint of all accesses.

Other than that CacheFS should preserve the same permissions semantics as if a given user/host were accessing the backend filesystem/service directly.

CacheFS & Security

hppnq — Fri, 03 Sep 2004 19:49:02 +0000

Not knowing anything about CacheFS internals, I would say these are cases of "don't do it, then". ;-)

CacheFS & Security

scripter — Thu, 02 Sep 2004 16:41:32 +0000

I wonder what the security implications of CacheFS are. Does each file inherit the permissions of the original? Is confidentiality a problem? What if you want to securely erase a file?